One example of a container of the kind specified is a can of the so-called "open-top" kind, i.e. a can comprising a can body which by itself has an open top end, but which has this end closed by a can end member secured to the can body by means of a peripheral double end seam, and which may also be of the kind including a can body cylinder having a longitudinal side seam. Another example is an aerosol can comprising a can body, which may be formed in one piece, or which may comprise a can body cylinder closed at its bottom end by an end member and at its top end by a domed cover member. The one-piece aerosol can body, or the domed cover member, has a mouth which is itself closed by a valve cup, carrying the aerosol dispensing valve. The valve cup is usually swaged on to the body.
As far as open-top cans are concerned, it has for many years been conventional practice to stamp a can end member from a sheet of metal which has been pre-lacquered for subsequent protection of the metal, or of the eventual contents of the can, or both, and to apply a suitable sealing compound to a peripheral flange of the can end member. Following this, the end member is positioned over an open end of the sheet metal can body, which is also pre-lacquered, with the peripheral flange of the end member overlying a peripheral flange of the body. The two flanges are then deformed together to produce a double seam.
This process has a number of disadvantages. Firstly, during the seaming operation there is a danger that the lacquer may be damaged on either the can end member or the can body as a result of local high pressure between the end member and the body, or friction between one of these parts and the seaming tools. If the lacquer is damaged there follows a risk of corrosion of the metal and of contamination of the contents of the can. Another problem is that the sealing compound is occasionally squeezed out during formation of the double seam and this again may have a detrimental effect on the quality of the seal provided by the seam and on the eventual contents of the can.
Reverting to aerosol cans, the same problems may also occur when the cover member is joined to the can body cylinder. Both of these parts may be lacquered prior to being joined together, and, as in the case of an open top can end member, a peripheral flange of the aerosol can cover member is lined with a suitable sealing compound. In this case, if the lacquer on an internal surface is damaged whilst the cover member is being secured to the can body, there is a considerable risk of internal rusting if the aerosol formulation to be contained by the can includes water. Additionally, if sealing compound is squeezed into the interior of the can body whilst the cover member is being secured to the latter, and particles of the compound become dislodged, they may eventually, in use, block the aerosol dispensing valve.
Similar problems may occur when the cup is swaged on to the cover member. The cup has a peripheral flange or cup curl, and between the cup curl and the body curl with which it forms a seam, there is interposed a sealing gasket. This gasket takes one of two forms, viz. a separate, thin washer-like member, or a layer of a sealing compound, applied to the underside of the cup curl in liquid form and then cured to a resilient, solid condition.
The sealing compound is typically of a suitable latex preparation, typically applied as a water-based suspension in sufficient quantities to give a final dry weight of 570 mg., corresponding to a dry thickness which at the thickest cross-section of the gasket is in the approximate range 0.50 to 0.65 mm. Apart from the problem, mentioned above, of pieces of the gasket possibly breaking off and falling into the contents of the container, this relatively great thickness of gasket material (lining compound) has another disadvantage. Although it is technically feasible to allow the wet latex suspension to dry naturally at ambient temperature, the storage time involved would be economically unacceptable. It is therefore necessary to accelerate drying, and to this end the provision of ovens is required. This, although cheaper than natural drying, is still very costly in terms of capital cost, maintenance cost, energy consumption and space requirements. Similar gaskets are commonly applied to the inside surface of one of the overlapping edge portions of an open-top can side or end seam.
There has for some time, unconnected with the problems discussed above, been considerable interest in laminated materials. These are being developed primarily to give them resistance to the temperatures employed in the "processing" (e.g. pasteurising) of foodstuffs or beverages packed in cans, as an alternative to the use of a tin coating, since this coating is becoming more and more expensive. The laminates concerned comprise a thin polymeric layer overlaid upon a metallic substrate. The base material used for such laminating may be for example "tin-free steel", blackplate, or aluminium. Out of many possible polymer films tested, polypropylene is one which appears promising for the packaging industry, due to its low cost, fusibility (faces can be heat sealed to each other), low extractability and ability to withstand processing temperatures. The back of the film may be printed prior to lamination, thus protecting the printing inks. Also, boxes such as biscuit boxes and the like may be completed by heat fusing at the joints after being folded.
Such laminates are quite well documented in the prior art, for the purposes mainly of providing a temporary surface having a low friction in order to facilitate working of the metal, or of rendering a tin coating on tinplate unnecessary having regard to the increasing cost of metallic tin.
Many proposals have been published in the patent literature for seams, for metal cans and other containers, in which the overlapping edge portions are bonded together. Sometimes the proposal is that this be achieved by means of an interposed adhesive compound; in other publications it has been proposed to apply polymeric layers, by way of local lamination, to the mating surfaces of the seam. In all of these proposals the use of heat is necessary to cure the adhesive compound or to cause the local polymeric surfaces to become fused together within the seam. In all of these cases the seam appears to present an effective seal by virtue of its two parts being bonded together. One proposal which is a variation on the above, relates to an aerosol can of the rather specialised kind in which the product to be dispensed is contained in a plastics bag or membrane within the can, to separate the product from the customary propellant. In that proposal the top edge of the bag is trapped within the valve cup seam, i.e. between the peripheral terminal curls or flanges of the valve cup and can body. However, in order to produce an effective seal within the seam, heat must be applied after the seam has been formed, in order to soften the plastics material within it.